Laser arrays are promising light sources for a number of applications such as projection displays and specialty lighting. Compared to lamps or light emitting diodes, for example, diode laser arrays offer tremendously increased brightness with long life and high reliability. Light generated by laser arrays can have high enough brightness to take advantage of nonlinear optical processes that convert light from one wavelength to another. For example, second harmonic generation (also called “frequency doubling”) is a nonlinear optical process by which light at infrared wavelengths can be converted to visible wavelengths.
One way to make visible light is to focus a high intensity infrared laser beam onto a suitable nonlinear optical material as first demonstrated in 1961 by Franken. More recently scientists have shown that second harmonic light may be generated by placing a nonlinear material inside a laser cavity; i.e. between two mirrors that form the ends of a laser. Second harmonic generation inside a laser offers higher conversion efficiency than can be achieved externally. An example of a device using this approach is the Novalux, Inc. (Sunnyvale, Calif.) Novalux Extended Cavity Surface Emitting Laser (NECSEL) array that produces light at red, green and blue wavelengths via intra-cavity second harmonic generation.
Surface emitting diode lasers emit light perpendicular to the surface of the semiconductor substrate on which they are fabricated. The laser structure includes a high reflector built on the substrate. An optical gain section is built between the high reflector and an output coupler. When the laser is designed for intra-cavity harmonic generation the output coupler is made to be a high reflector at the fundamental frequency and transparent at the harmonic frequency.
Consider the situation in which second harmonic light is generated inside a diode laser cavity in a nonlinear optical material. The high reflector on the substrate is likely to be transparent at the second harmonic wavelength. Second harmonic light passing through the output coupler can be easily directed to the application at hand. Second harmonic light passing through the substrate high reflector, however, is lost to absorption in the semiconductor substrate.
Second harmonic light traveling toward the substrate high reflector can be saved by inserting a dichroic beam splitter in the laser cavity. A dichroic beam splitter reflects light at the second harmonic wavelength while transmitting light at the fundamental laser wavelength. Mirrors external to the laser cavity may be used to direct the second harmonic light in any direction, including quite usefully, parallel to the second harmonic light that passed through the laser output coupler. In this way as much visible light as possible may be extracted from an infrared diode laser array featuring intra-cavity second harmonic generation.
Although the method of extracting second harmonic light from a diode laser cavity through an output coupler in one direction and a dichroic beam splitter in the opposite direction is effective in terms of total light power recovered, it may not be optimum when beam focusing is considered. Light from the two sources is focused at different planes when both are focused by a single lens. The difficulty comes from the two different path lengths traveled by second harmonic light from its waist inside the laser cavity. Depending upon how external mirrors are arranged, the path via the output coupler to a focusing lens may be shorter than the path via the dichroic beam splitter.
Surface emitting diode laser arrays with intra-cavity second harmonic generation would benefit from an optical system that not only extracted second harmonic light in both directions from the intra-cavity nonlinear optical material, but did so in such a way that the focusing properties of both light beams were matched.